One of the most important trainings in music education is to improve musicianship of students. Musicianship includes the capabilities of singing and playing music instrument with correct frequencies and timing, counting beat correctly and finding characters of music by hearing. These are also one of the major subjects in music examinations provided by international music institutes, such as Associate Board of Royal School Music (ABRSM) in UK, Royal Conservatory of Music Examinations (RCM) in Canada, Australian Music Examinations Board (AMEB) in Australia and etc.
Previously, all these trainings can only be provided by teachers within a room. While there are some other materials, e.g. recorded waves in CD or online to train students in finding characters of music by hearing, there is no way for students to learn singing, playing a music instrument, and counting beats without the teacher as the performance must be judged by the teacher and the teacher will give feedback after listening to the student's performance. Even though students can record the performance and send the recording to the teacher though a network, it still requires a teacher to judge.
On the other hand, in any teaching with a teacher, the teacher can only tell what the good points and bad points about a student's performance are by mouth. The teacher can record the performance and replay it. However, it is very inconvenient for the teacher to record the performance every time. Even when there is a recording, the student needs to imagine which part of the performance is good or bad based on teacher's words, without any visual aids to help.
In addition, since the performance of the students can vary in many directions, the teacher cannot record the performance in a scientific way. In other words, the teacher can only mark some major errors but it is difficult for the teacher to have a whole picture on the progress of the students' performance.